The chairman of the congressional Joint Economic Committee yesterday accused the Reagan administration of "immoral and illegal" action in deciding to reserve a U.S. policy of opposing loans by international development banks to four South American military regimes.

"The administration has played fast and loose with the law," Rep. Henry S. Reuss (D-Wis.) told Myer Rashish, undersecretary of state for economic affairs.

Reuss contended that the decision to support loans or Argentina, Chile, Uruguay and Paraguay violates a 1977 law barring U.S. backing for such loans to countries engaged in systematic violations of human rights.

Similar criticism of the administration's action was expressed to Walter J. Stoessel Jr., undersecretary for political affairs, by Rep. Don Bonker (D-Wash.), chairman of the House subcommitte on human rights, during a hearing yesterday on the administration's rights policy.

In a turnabout from the policy set by former president Carter, the administration decided July 1 that it no longer will abstain or vote against loans from the World Bank and the Inter-American Development Bank to the four countries. Secretary of State Alexander M. Haig Jr. justified the move on the grounds that these regimes had improved their rights records, and both Stoessel and Rashish reiterated that position yesterday.

In a lengthy statement, Stoessel stressed that the administration intends to press ahead with abandoning the activist stance of the Carter years in favor of a "quiet diplomacy" approach.

The Reagan administration has said it will differentiate between "totalitarian" regimes like the Soviet Union and "authoritarian" governments that cooperate with U.S. diplomatic and strategic goals. These distinctions have caused considerable controversy, and Stoessel did not use them in his statement yesterday.

Under questioning, he admitted that a decision has been made to avoid talking about "totalitarian" and "authoritarian" governments because the administration, while still believing the distinction is valid, feels the controversy has caused confusion about President Reagan's commitment to even-handedness in the human rights area.

He said: "As the spokesman for the American people, this administration opposes the violation of human rights whether by ally or adversary, friend or foe. Ours is not a policy of 'selective indignation.' Rather, it is one of balanced and even-handed condemnation of human rights violations wherever they occur."

But he strongly defended the emphasis, in dealing with friendly countries, on speaking to them "privately at first and in ways which do not threaten them with public loss of face. . . . We should speak to them, where possible, in the framework of friendly relations grounded in trust and reciprocity."

Rep. Tom Harkin (D-Iowa), disputing the idea that quiet diplomacy is more effective, revealed the findings of an opinion poll conducted for the International Communication Agency earlier this year in nine Latin American countries. The poll results, which have been circulated within the government but not made public, indicated that in almost all of the countries surveyed, the respondents believed the United States should put strong emphasis on protecting human rights, while few respondents thought the United States should increase military aid to the region.